Stabilizers or secondary emulsifiers are well known in the latex art. They are added to the emulsifier or soap solution used in the emulsion polymerization in order to reduce the formation of coagulum or gum. Other variables such as choice of agitation conditions can influence latex stability as well by affecting particle size and distribution.
Various types of stabilizers have been used. Protective colloids, exemplified by methacrylates, gelatin, pectinates, and caseinates, are frequently used in combination with emulsifiers to effect micelle concentration or to prevent premature coagulation by forming a thin film at the interface between the polymer droplets and the water.
Secondary emulsifiers, generally used at concentrations between 0.04 and 1 weight percent based on total monomer being 100 weight percent, are divided into three groups. Anionic soaps are exemplified by alkali salts of palmitic acid, stearic acid and oleic acid. Nonionic soaps, for example, reaction products of long-chain alcohols with ethylene oxide, are resistant to cold temperatures and changes in pH. They are expensive, make coagulation difficult, and require addition of other materials such as amines for pH control and to aid in coagulation. Cationic soaps are frequently used in the latex industry and are exemplified by dodecylammonium chloride and hexadecylpyrridinium chloride.
Electrolytes such as potassium chloride and sodium phosphate are also used in the soap solutions. The presence of an electrolyte lowers the latex viscosity and affects particle size. However, too much electrolyte can cream the latex or cause it to flocculate.
High molecular weight alcohols and polyhydroxy compounds can serve as stabilizers in certain cases.
One widely used dispersant stabilizer is the sodium salt of a condensed naphthalene sulfonic acid. It is readily soluble in water and is not appreciably affected by salts and acids although it is most effective at alkaline pH. Most latices are manufactured at a pH ranging from 9 to 12.
In using the sodium salt of a condensed naphthalene sulfonic acid in rubber latex manufacture, care must be taken in choosing the level at which it is used. The quantity should be sufficient to accomplish the desired goal of reducing prefloc or gum but not so great that it interferes with coagulation. In fact, it is such an effective dispersant that it causes problems in the treatment of waste water effluent from rubber manufacturing facilities.
Primary waste water treatment in these factories often consists of coagulation followed by sedimentation or flotation which processes serve to remove a substantial proportion of pollutants (suspended solids, chemical oxygen demand, COD, and biological oxygen demand, BOD). The sodium salt of a condensed naphthalene sulfonic acid is not affected by the rubber coagulation process and remains in the supernatant spent coagulant liquor which is rejected to the waste treatment process. Unfortunately, it is also unaffected by the coagulants used therein, and it resists biodegradation. It remains dissolved and continues through the rest of the waste treatment steps to the final discharge at least partially intact. It is a significant contributor to COD concentration, and a reduction in effluent COD is the constant goal of those working in the rubber industry.
The stabilizers of this invention, while doing an excellent job of dispersing the emulsified rubber droplets prior to the coagulation step, do not become part of the process effluent. Instead, they are precipitated in the rubber coagulation process with the rubber crumb.
Another stabilizer which stays with the rubber after coagulation is described by U.S. Pat. No. 3,544,502 (salts of polymethylated muconic acids). However, these are added after the polymerization reaction, not as part of the reaction emulsifier solution.
Copolymers and adducts containing maleic acid esters and maleic anhydride are disclosed in the following references: U.S. Pat. No. 2,912,399 (copolymers of maleic acid esters and styrene in an organic solvent as an ingredient in waterproof emulsions); U.S. Pat. No. 2,871,137 (salt of an adduct of maleic anhydride with SRR, NBR or another rubber as an emulsifying agent); Encyclopedia of Polymer Science and Technology, Vol. 5, p. 822, Interscience Publ., N.Y. 1966 (styrene-maleic anhydride copolymers as a protective colloid in emulsion polymerizations).